Integrated optics technology is becoming increasingly refined and with the advent of thin film optical systems, it has become desirable to fabricate thin film lenses. The step-in-thickness type of thin film lens is very nearly the equivalent of a conventional cylindrical lens, only in thin form. This type of thin film lens can be fabricated using photolithographic processes by plasma etching through a photoresist mask in such optical waveguide materials as Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5. A good quality of the plasma etching process is that plasma etching through a photoresist mask results in an etch wall with steep smooth sides. A limitation of this approach, however, is that the etch rate is too variable for reproducible etch depth based upon etch time and the bottom of the etched area is very rough causing excessive scattering loss.
In the present invention a three layer optical waveguide is provided by introducing a layer of TiO.sub.2 intermediate two main layers of Nb.sub.2 O.sub.5. Although both Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 are good waveguide materials in this invention, Nb.sub.2 O.sub.5 is preferred over Ta.sub.2 O.sub.5 because its refractive index is greater. A good control of the etch depth and a smooth flat bottom in the etch areas have been achieved by the layer of TiO.sub.2 acting as an etch stop.